Reference Guide

For example, if you configure TPID 0x9100, the system treats 0x8100 and untagged traffic the same and maps both types to the
default VLAN, as shown by the frame originating from Building C. For the same traffic types, if you configure TPID 0x8100, the
system is able to differentiate between 0x8100 and untagged traffic and maps each to the appropriate VLAN, as shown by the
packet originating from Building A.
Therefore, a mismatched TPID results in the port not differentiating between tagged and untagged traffic.
Figure 108. Single and Double-Tag TPID Match
690
Service Provider Bridging